1. Field of the Invention
The invention relates to a system for monitoring or supervising the repeaters of an optical signal transmission installation.
2. Description of the Prior Art
The transmission of signals over optical fibers is now a routine technique in diverse applications, in particular in submarine telephone lines. Its advantages include the ability to accommodate high bit rates with a high bandwidth without significant deterioration of the signals.
An installation of this type comprises a transmit center in which there is provided at least one laser source the luminous energy from which is modulated by the signal to be sent; the modulated energy is transmitted by optical fibers to at least one remotely located receiver. Between the transmitter and the receiver there are repeaters in the form of optical amplifiers such as doped fiber optical amplifiers. The function of the repeaters is to amplify the signal periodically, attenuation of the signal being inherent to the transmission medium, so that it reaches the receiver at a satisfactory level.
The most elaborate installations include branch connections to secondary receivers. Each branch connection, also using optical fibers, also includes one or more repeaters.
For example, a transatlantic optical link comprises a transmitter in North America and its main receiver in a European country. The branch connections go to other European countries in which secondary receivers are located.
In a complex high bit rate installation several different transmission optical wavelengths, i.e. several lasers, may be used.
The correct functioning of an installation of this kind is largely dependent on the correct functioning of the repeaters. It is therefore necessary to monitor these devices periodically or continuously. However, as they are widely dispersed and often at locations where access is difficult, remote monitoring is used. To this end, a remote monitoring signal is transmitted over the line from the transmit center, often superimposed on the signal carrying wanted information. The remote monitoring signal generally has a bit rate of 1 kbit/s, while the wanted signal has a bit rate of 5 Gbit/s. Its level is usually low in comparison to that of the wanted signal.
Remote monitoring is effected by means of a protocol for interrogating each repeater. Each of the latter is identified by its numerical address. For example, the interrogation protocol entails detecting the input power, the output power and the power of the pump laser of the optical amplifier. The response signals are retransmitted to the transmit center by the repeater using the return channel of the line. In a simpler system the interrogation protocol is limited to detecting the output power of each repeater.
In a single-channel transmission system, i.e. one using only one optical wavelength, the remote monitoring signal is superimposed on the wanted signal, i.e. the wanted signal is modulated by the remote monitoring signal. The depth of modulation is usually in the order of 5%. This value represents a compromise between the need to reduce the disturbance of the wanted signal and the need to transmit the remote monitoring signal correctly.
In an installation using a plurality of optical wavelengths and with branch connections it is preferable not to modulate individually each of the wavelengths used to transmit the wanted signals. In installations of this kind, some wavelengths being assigned to transmission to branch connections, the remote monitoring signal would be lost downstream of the branch connection.
It is true that a secondary center in a branch connection retransmits a wanted signal; however, it is preferable not to superimpose a monitoring signal on the retransmitted wanted signal in order to reserve the monitoring function to the main center.
Under the above conditions the signal at the output of the repeater downstream of the branch connection would be modulated with a depth of modulation less than the optimal value of 5%. Each repeater transmits a constant output power, so it is understandable that if the repeater transmits two wavelengths of which one carries no modulation and the other carries 5% modulation, the output signal will be modulated to a depth of 2.5% if the optical powers of the two wavelengths are equal.
Furthermore, as the gain of the optical amplifiers is not the same for all wavelengths, so it is also understandable that the reduction in the modulation will be even more significant if the wavelength extracted and re-injected with no remote monitoring signal corresponds to the maximal value of the gain of the optical amplifiers.
The invention remedies these drawbacks. It also makes the disturbance due to the remote monitoring modulation virtually negligible.